The goal of this research is to examine semantic memory impairments associated with DAT. The research focuses on the cognitive and linguistic causes of the impairments in object naming and in the comprehension of object names that are characteristic of the disease. The theoretical framework for this work is provided by connectionist models of the lexicon, which address the representation of lexical knowledge and the computation of lexical codes in comprehension and production. This framework provides a basis for clarifying the distinction between deficits in semantic representation and in the "access" of semantic information that has been the focus of previous research. The comprehension and use of object names is seen as a complex skill with multiple components that can be individually impaired, resulting in different degrees of impairment and types of errors. The core of the project is a series of behavioral studies examining detailed aspects of patients' use of object names and their knowledge of concepts and categories, focusing on impairments in different types of features or attributes and in their frequencies and intercorrelations. The principal goal is to develop an account of how different types of damage to the normal system result in patients' word-finding and comprehension deficits, and systematic object naming errors. The data will also be used as part of a longer-term project in which the goal is to develop an account of lexical deficits in DAT over the course of the disease within a general computational model of lexical processing.